1. Field of the Invention
The present invention relates to a digital image forming apparatus forming an electrostatic latent image and visualizing the same with toner, and more particularly, it relates to an image forming apparatus attaining improvement in gradation, resolution and stability.
2. Description of the Prior Art
In relation to a copying machine or a printer of an electrophotographic system, resolution of a digital printing technique is improved in order to satisfy requirement for high image quality. While the mainstream resolution has generally been 400 to 600 DPI (dots per inch), an image forming apparatus having resolution of at least 1200 DPI is now being put into practice. In order to implement such high resolution, the technical level must be improved in each process of the electrophotographic system. It is argued that reduction of an exposure beam diameter, reduction of the particle size of toner, reduction of the thickness of a photoreceptor and the like are qualitatively effective for such improvement.
Japanese Patent Laying-Open No. 58-152269 (1983) discloses exemplary numerical limitation of 1.8 p.ltoreq.W.ltoreq.1.9 p as to the relation between a minimum dot recording period p (inverse number of resolution) and an exposure beam diameter W. More specifically, Journal of Society of Electrophotography of Japan Vol. 26, No. 4, pp. 24 to 33 (1987) argues the optimum spot diameter of an exposure beam and discloses exemplary numerical limitation of 1.3 p&lt;W&lt;1.6 p while the value varies with laser power. The minimum dot recording period p indicates the minimum period of dots forming an image. For example, the minimum dot recording period p of a printer having resolution of 600 DPI is 25.4/600=0.0423 mm.
Japanese Patent Laying-Open No. 9-319164 (1997) discloses that deterioration of a latent image can be suppressed without employing a thin-film photoreceptor by limiting the relation between an exposure beam diameter, the thickness of a photoreceptor and a contrast potential of a latent image. Further, Japanese Patent Laying-Open No. 10-282709 (1998) discloses that deterioration of a latent image as well as deterioration of transfer can be suppressed by limiting the relation between the thickness of a photoreceptor, a transfer current and the area of an exposure beam. Similarly, Japanese Patent Laying-Open No. 5-216330 (1993) limits a film thickness, a beam diameter, the particle size of toner and the diameter of a carrier, while Japanese Patent Laying-Open No. 10-171221 (1998) limits a film thickness, a beam diameter, exposure energy and sensitivity of a photoreceptor. In addition, "Ion Printing Technology" in Journal of Imaging Technology, Vol. 12, 144 (1986) discloses a technique of directly supplying ions onto an insulator for forming an electrostatic latent image dissimilarly to that employing the aforementioned photoreceptor or an exposure device, and implements image formation with resolution of 300 DPI.
As described above, the resolution has recently been improved from 600 DPI to 1200 DPI, in order to satisfy the requirement for high image quality in a copying machine or a printer. Under the present circumstances, however, true resolution is not yet improved due to various practical problems.
For example, a conventional apparatus setting only resolution to 1200 DPI with an exposure beam of about 70 to 80 .mu.m in diameter for 600 DPI cannot implement an isolated dot or an isolated line of 1200 DPI. This is because the contrast of a latent image potential is so reduced that field strength necessary for development cannot be formed when the resolution of the latent image is improved. The isolated dot is an image pattern of a single black or white dot on a white or black background. The isolated line is an image pattern of a single black or white line on a white or black background. White indicates a recording medium part (paper or the like) recording no image, and black indicates a part (color part) recording an image.
In order to improve resolution with a photoreceptor having a general thickness, a method of reducing the diameter of an exposure beam and increasing a contrast potential may be applied as shown in Japanese Patent Laying-Open No. 9-319164. The contrast potential defined herein is the difference between an initial charge potential of a photoreceptor and a charge potential in full exposure. In view of frequency characteristics of a latent image, the contrast potential indicates the contrast of a latent image potential with respect to a DC latent image. The contrast of the latent image potential is reduced as the spatial frequency (hereinafter referred to as a latent image spatial frequency) of the latent image is increased. In a high-frequency latent image formed with resolution of at least 600 DPI, particularly with resolution of at least 1200 DPI, the contrast of the latent image potential is remarkably reduced to result in various types of image quality deterioration. The latent image spatial frequency is expressed in line pairs/mm as the number of pairs of white and black lines having the same line width existing in an area of 1 mm.
While a latent image can be beautifully formed when simply reducing the thickness of a photoreceptor or the diameter of an exposure beam, it is extremely difficult to develop the latent image. This is because the latent image frequency is increased regardless of the beauty/fidelity of the latent image and hence its potential amplitude is so reduced that field strength necessary for development cannot be formed. Further, reduction of the thickness of the photoreceptor must be kept at the minimum necessary level so that the life of the photoreceptor is not reduced due to abrasion.
In addition, compatibility of a periodic line pattern (.lambda.=2 p) of a minimum on/off recording period .lambda.[.mu.m], an isolated online pattern and an isolated offline pattern is disadvantageously deteriorated following improvement of the resolution. The periodic line pattern of the minimum on/off recording period is an image pattern periodically forming presence/absence of dots in the minimum dot recording period p. The isolated online pattern is an image pattern forming a black line (ine of a color other than black in the case of a color printer) on a white background. The isolated offline pattern is an image pattern forming a white line on a black background (background of a color other than black in the case of a color printer).